Resolve technical debt in stages_dropdown.vue

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Resolve technical debt in app/assets/javascripts/jobs/components/stages_dropdown.vue

• In isMergeRequestPipeline flags should be present if we can check for merge_request_pipeline. We should be able to just return this.pipeline?.flags?.merge_request_pipeline and it should always already be a Boolean so no need to convert.
• We need to check isTriggeredByMergeRequest I'm not seeing merge_request present on the pipeline object. Maybe we can just check this.pipeline?.merge_request without converting to a Boolean. Or we can check with backend.
• Let's use data-testid instead of the legacy js classes for testing
• Let's use gitlab-ui utility classes